Lidar, of course, doesn't actually see through vegetation. Rather, it goes through the holes in the foliage. Some of the multiple laser pulses it emits simply find openings between leaves and branches, in the same way that sunlight filters through the forest canopy and continues to the ground. In this process, the LiDAR unit sends a pulse to the vegetation and sends a second pulse before the first one reaches the ground.
The unit is then controlled in such a way that when the first beam returns, it collides with the second beam that has just started its journey from the scanner.